Method of determining axial alignment of the centroid of an edge gripping end effector and the center of a specimen gripped by it

ABSTRACT

A method determines axial alignment between the centroid of an end effector and the effective center of a specimen held by the end effector. The method is implemented with use of an end effector coupled to a robot arm and having a controllable supination angle. A condition in which two locations of the effective center of the specimen measured at 180° displaced supination angles do not lie on the supination axis indicates that the centroid is offset from the actual effective center of the specimen.

RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.10/649,116, filed Aug. 26, 2003, now U.S. Pat. No. 6,898,487, which is adivision of U.S. patent application Ser. No. 10/223,075, filed Aug. 15,2002, now U.S. Pat. No. 6,618,645, which is a division of U.S. patentapplication Ser. No. 09/920,353, filed Aug. 1, 2001, now U.S. Pat. No.6,438,460, which is a division of U.S. patent application Ser. No.09/312,343, filed May 14, 1999, now U.S. Pat. No. 6,275,748, which is acontinuation-in-part of U.S. patent application Ser. No. 09/204,747,filed Dec. 2, 1998, now U.S. Pat. No. 6,256,555.

FIELD OF THE INVENTION

This invention is directed to a specimen handling apparatus and methodand, more particularly, to a method of determining the axial alignmentof the centroid of a semiconductor wafer robot arm edge gripping endeffector to the center of a semiconductor wafer gripped by the endeffector.

BACKGROUND OF THE INVENTION

Integrated circuits are produced from wafers of semiconductor material.The wafers are typically housed in a cassette having a plurality ofclosely spaced slots, each of which can contain a wafer. The cassette istypically moved to a processing station where the wafers are removedfrom the cassette, placed in a predetermined orientation by a prealigneror otherwise processed, and returned to another location for furtherprocessing.

Various types of wafer handling devices are known for transporting thewafers to and from the cassette and among processing stations. Manyemploy a robotic arm having a spatula-shaped end that is inserted intothe cassette to remove or insert a wafer. The end of the robotic arm isreferred to as an end effector that typically employs a vacuum toreleasibly hold the wafer to the end effector. The end effectortypically enters the cassette through the narrow gap between a pair ofadjacent wafers and engages the backside of a wafer to retrieve it fromthe cassette. The end effector must be thin, rigid, and positionablewith high accuracy to fit between and not touch the closely spaced apartwafers in the cassette. After the wafer has been processed, the roboticarm inserts the wafer back into the cassette.

Unfortunately, transferring the wafer among the cassette, robot arm, andprocessing stations, such as a prealigner, may cause backside damage tothe wafer and contamination of the other wafers in the cassette becauseintentional engagement as well as inadvertent touching of the wafer maydislodge particles that can fall and settle onto the other wafers. Waferbackside damage can include scratches as well as metallic and organiccontamination of the wafer material. Robotic arms and prealigners thatemploy a vacuum to grip the wafer can be designed to minimize backsidedamage and particle creation. Even the few particles created with vacuumpressure gripping or any other non-edge gripping method are sufficientto contaminate adjacent wafers housed in the cassette. Reducing suchcontamination is particularly important to maintaining wafer processingyields. Moreover, the wafer being transferred may be scratched orabraded on its backside, resulting in wafer processing damage.

What is needed, therefore, is a specimen gripping end effector that cansecurely, quickly, and accurately transfer semiconductor wafers whileminimizing wafer scratching and particle contamination.

SUMMARY OF THE INVENTION

An object of this invention is, therefore, to provide a specimenhandling device that minimizes specimen damage and the production ofcontaminant particles.

Another object of this invention is to provide a semiconductor waferhandling device that can quickly and accurately transfer semiconductorwafers between a wafer cassette and a wafer processing station.

A further object of this invention is to provide a wafer handling devicethat can be retrofit to existing robot arm systems.

Robot arm end effectors of this invention rapidly and cleanly transfersemiconductor wafers between a wafer cassette and a processing station.Embodiments of the end effectors include at least one proximal rest padand at least two distal rest pads having pad and backstop portions thatsupport and grip the wafer at its peripheral edge or within an annularexclusion zone that extends inward from the peripheral edge of thewafer. The end effectors also include an active contact point that ismovable between a retracted wafer-loading position and an extendedwafer-gripping position. The active contact point is movable to urge thewafer against the distal rest pads so that the wafer is gripped only atits edge or within the exclusion zone. The end effectors are configuredso that wafer edge contact is achieved for end effectors with inclinedrest pads. Optical sensors detect retracted, safe specimenloading/gripping, and extended positions of the active contact point.

The end effectors are generally spatula-shaped and have a proximal endthat is operably connected to a robot arm. The active contact point islocated at the proximal end, which allows the end effector to belighter, stronger, and more slender than end effectors having movingmechanisms that may not fit between adjacent wafers in a cassette. Thelack of moving mechanisms that could be located over a wafer furthercauses the end effector to produce less contamination within thecassette. Additionally, locating the active contact point at theproximal end of the end effector ensures that it is remote from harshconditions such as heated environments and liquids.

A vacuum pressure-actuated piston moves the active contact point betweena retracted position, in which the wafer is loaded into the endeffector, and an extended position in which the wafer is gripped. Afirst embodiment of the piston employs vacuum pressure to move theactive contact point between extreme positions; a second embodiment ofthe piston employs vacuum pressure to retract the active contact pointand a spring to extend the active contact point; and a third embodimentof the piston adds the above-mentioned optical sensors for detectingretracted, safe specimen loading/gripping, and extended positions of theactive contact point.

Alternative embodiments of the end effector include flat or inclined,narrow or arcuate rest pads onto which the wafer is initially loaded.The narrow and arcuate inclined rest pad embodiments assist in centeringand gripping the wafer between the active contact point and the distalrest pads. The arcuate rest pads more readily accommodate gripping andhandling flatted wafers.

Embodiments of the end effectors further include fiber optic lighttransmission sensors for accurately locating the wafer edge and bottomsurface. Three alternative embodiments include placing the wafer edgeand bottom sensors at the proximal end of the end effector; placing theedge sensors at the proximal end and the bottom sensors at the distalend of the end effector; and placing a combined edge and bottom sensorat the distal end of the end effector. In all three embodiments, thesensors provide robot arm extension, elevation, and positioning datathat support methods of rapidly and accurately placing a wafer on andretrieving a wafer from a wafer transport stage or a process chamber,and placing a wafer in and retrieving a wafer from among a stack ofclosely spaced wafers stored in a wafer cassette. The methodseffectively prevent accidental contact between the end effector andadjacent wafers stacked in a cassette or a wafer resting on a processingdevice while effecting clean, secure gripping of the wafer.

Additional objects and advantages of this invention will be apparentfrom the following detailed description of preferred embodiments thereofwhich proceed with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of a first embodiment of the end effector of thisinvention shown inserted into a semiconductor wafer cassette to retrieveor replace a wafer.

FIG. 2 is a side elevation view of the end effector of FIG. 1 withoutthe wafer cassette but showing the end effector inserted between anadjacent pair of three closely spaced apart wafers as they would bestored in the cassette.

FIG. 3 is an enlarged side elevation view of a flat rest pad embodimentof this invention showing the rest pad engaging an exclusion zone of awafer.

FIG. 4 is an enlarged side elevation view of an inclined rest padembodiment of this invention showing the inclined rest pad engagingsubstantially a periphery of a wafer.

FIG. 5 is a fragmentary plan view of a portion of the end effector andwafer of FIG. 1, enlarged to reveal positional relationships among thewafer and a movable contact point, wafer rest pads, and wafer edge andelevation sensors of the first embodiment end effector of thisinvention.

FIGS. 6A and 6B are respective side and front elevation views of one ofthe edge and elevation sensors of FIG. 5, further enlarged to reveal thepositioning of fiber optic light paths relative to the wafer.

FIG. 7 is a plan view of a second embodiment of the end effector of thisinvention shown gripping a semiconductor wafer and adjacent to asemiconductor wafer in a wafer cassette to sense, retrieve, or replace awafer.

FIG. 8 is a sectional side elevation view of the end effector of FIG. 7showing an active contact point actuating mechanism gripping a waferbetween adjacent ones of closely spaced apart wafers as they would bestored in the wafer cassette.

FIG. 9 is an enlarged isometric view of a distal arcuate rest padembodiment of this invention mounted on the distal end of the endeffector of FIG. 7.

FIG. 10 is an end perspective view of the end effector of FIG. 7 showingpositional relationships among the movable contact point, arcuate restpads, and wafer edge and elevation sensors of the second embodiment endeffector of this invention.

FIG. 11 is a bottom view of the end effector of FIG. 7 showing fiberoptic routing channels for elevation sensors of the second embodimentend effector of this invention.

FIG. 12 is a fragmentary plan view of a portion of a third embodiment ofan end effector of this invention, showing positional relationshipsamong the wafer, a position sensing active contact point actuatingmechanism, and the proximal rest pads.

FIG. 13 is a sectional side elevation view of the end effector portionof FIG. 12 showing the position sensing active contact point actuatingmechanism fully extended between adjacent closely spaced wafers as theywould be stored in the wafer cassette.

FIG. 14 is an overall plan view of the end effector of FIG. 12 showingalternate wafer gripping and sensing positions.

FIGS. 15A and 15B are respective side elevation and plan views of anexemplary two-arm, multiple link robot arm system from which the endeffector of the present invention extends.

FIG. 16 is a side elevation view in stick diagram form showing the linkcomponents and the associated mechanical linkage of the robot arm systemof FIGS. 15A and 15B.

FIG. 17 is an isometric view in stick diagram form showing therotational motion imparted by the motor drive links of the mechanicallinkage of the robot arm system of FIGS. 15A and 15B.

FIG. 18A is a diagram showing the spatial relationships and parametersthat are used to derive control signals provided by, and FIG. 18B is ablock diagram of, the motor controller for the robot arm system of FIGS.15A and 15B.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIGS. 1 and 2 show a first embodiment of a spatula-shaped end effector10 of this invention for transferring semiconductor wafers, such as awafer 12 (shown transparent to reveal underlying structures), to andfrom a wafer cassette 14. End effector 10 is adapted to receive andsecurely hold wafer 12 and transfer it to and from cassette 14 forprocessing. FIG. 2 shows that end effector 10 is particularly adaptedfor retrieving and replacing wafer 12 from among closely spaced wafers,such as wafers 12, 12A, and 12B, which are shown as they might bestacked in slots 13 of wafer cassette 14, or from a lowermost slot 13 ofwafer cassette 14. Wafers having diameters of less than 150 mm aretypically spaced apart at a 4.76 mm ( 3/16 inch) pitch distance; 200 mmdiameter wafers are typically spaced apart at a 6.35 mm (¼ inch) pitchdistance; and 300 mm wafers are typically spaced apart at a 10 mm (0.394inch) pitch distance.

End effector 10 is operably attached to a robot arm 16 (a portion ofwhich is shown) that is programmably positionable in a well knownmanner. In general, end effector 10 enters wafer cassette 14 to retrievewafer 12 positioned between wafers 12A and 12B. End effector 10 is thenfinely positioned by robot arm 16 and actuated to grip a periphery 18 ofwafer 12, remove wafer 12 from cassette 14, and transfer wafer 12 to aprocessing station (not shown) for processing. End effector 10 may then,if necessary, reinsert wafer 12 into cassette 14, release wafer 12, andwithdraw from cassette 14.

End effector 10 is operably coupled to robot arm 16 at a proximal end 20and extends to a distal end 22. End effector 10 receives wafer 12between proximal end 20 and distal end 22 and includes on a supportsurface 10 s at least two and, preferably, four rest pads upon whichwafer 12 is initially loaded. Two distal rest pads 24 are located at, oradjacent to, distal end 22 of end effector 10; and at least one, butpreferably two proximal rest pads 26 are located toward proximal end 20.Distal rest pads 24 may alternatively be formed as a single arcuate restpad having an angular extent greater than the length of a “flat,” whichis a crystal structure-indicating feature commonly found onsemiconductor wafers. A flat 27 is shown, by way of example only,positioned between proximal rest pads 26. Of course, wafer 12 may have adifferent orientation, so periphery 18 is also shown positioned betweenproximal rest pads 26.

Wafer 12 includes an exclusion zone 30 (a portion of which is shown indashed lines). Semiconductor wafers have an annular exclusion zone, orinactive portion, that extends inwardly about 1 mm to about 5 mm fromperiphery 18 and completely surrounding wafer 12. Exclusion zone 30 isdescribed as part of an industry standard wafer edge profile template inSEMI (Semiconductor Equipment and Materials International) specificationM10298, pages 18 and 19. As a general rule, no part of end effector 10may contact wafer 12 beyond the inner boundary of exclusion zone 30. Itis anticipated that future versions of the specification may allow edgecontact only, a requirement that is readily accommodated by thisinvention.

The distance between rest pads 24 and the distance between rest pads 26each have an angular extent greater than any feature on wafer 12 toguarantee that wafer 12 is gripped only within exclusion zone 30. Restpads 24 and 26 may be made of various materials, but a preferredmaterial is polyetheretherketone (“peek”), which is a semi-crystallinehigh temperature thermoplastic manufactured by Victrex in the UnitedKingdom. The rest pad material may be changed to adapt to differentworking environments, such as in high temperature applications.

FIG. 3 shows a substantially flat embodiment of distal rest pads 24.This embodiment can be advantageously, but need not exclusively be, usedwith wafers having less than about a 200 mm diameter. Distal rest pads24 include a pad portion 32 and a backstop portion 34. In the flatembodiment, pad portion 32 is substantially parallel to an imaginaryplane 36 extending through wafer 12, and backstop portion 34 is inclinedtoward wafer 12 at a backstop angle 38 of up to about 5 degrees relativeto a line perpendicular to plane 36. Alternatively, pad portion 32 maybe inclined away from wafer 12 up to about 3 degrees relative to plane36. Pad portion 32 has a length 40 that is a function of the depth ofexclusion zone 30, but is preferably about 3 mm long. Wafer 12 typicallyhas a substantially rounded peripheral edge and contacts rest pads 24only within exclusion zone 30. Wafer 12 is gripped by urging it into theincluded angle formed between pad portion 32 and backstop portion 34.

FIG. 4 shows an inclined embodiment of distal rest pads 24. Thisembodiment can be advantageously, but need not exclusively be, used withwafers having greater than about a 200 mm diameter. Distal rest pads 24include an inclined pad portion 42 and a backstop portion 34. In theinclined embodiment, inclined pad portion 42 is inclined away from wafer12 at a rest pad angle 44 of about 3 degrees relative to plane 36, andbackstop portion 34 is inclined toward wafer 12 at backstop angle 38 ofup to about 3 degrees. Inclined pad portion 42 has a length 40 that is afunction of the depth of exclusion zone 30, but is preferably about 3 mmlong. As before, wafer 12 typically has a substantially roundedperipheral edge and contacts rest pads 24 only within exclusion zone 30.Wafer 12 is gripped by urging it into the included angle formed betweenpad portion 42 and backstop portion 34. In the inclined embodiment,there is substantially no contact between rest pad 24 and a bottomsurface 46 of wafer 12. This rest pad embodiment is also suitable forwafer edge contact only.

Both the flat and inclined embodiments of distal rest pads 24 have aheight 48 that substantially reaches but does not extend beyond the topsurface of wafer 12.

Referring again to FIG. 1, proximal rest pads 26 are similar to distalrest pads 24 except that each rest pad 26 does not necessarily require abackstop portion and its pad portion has a length of about twice that oflength 40.

End effector 10 further includes an active contact point 50 that islocated at proximal end 20 of end effector 10 and between proximal restpads 26. Alternatively, proximal end contact point 50 is formed as partof a proximal rest pad 26. Active contact point 50 is movable between aretracted wafer-loading position (shown in dashed lines) and an extendedwafer-gripping position (shown in solid lines). As shown, in theretracted position, active contact point 50 is positioned between andbehind front side margins 28 of the pad portions of proximal rest pads26.

Active contact point 50 is operatively connected to a piston 52 forreciprocation between the retracted and extended positions. In a firstembodiment, piston 52 reciprocates within a bore 54 and is preferablyvacuum pressure operated to extend and retract active contact point 50.Active contact point 50 is connected to piston 52 by a piston rod 56that extends through an airtight seal 58. Bore 54 forms a vacuum chamberin end effector 10 that is divided by piston 52 into a drive chamber 60and a return chamber 62. Drive chamber 60 is in pneumatic communicationwith a vacuum pressure source (not shown) through a first channel 64,and return chamber 62 is in pneumatic communication with the vacuumpressure source through a second channel 66. The vacuum pressure actsthrough drive chamber 60 against the front face of piston 52 to extendactive contact point 50 to the wafer-gripping position and acts throughreturn chamber 62 against the back face of piston 52 to retract activecontact point 50 as controlled by the programmable control. The vacuumpressure source is routed to first and second channels 64 and 66 throughrotary vacuum communication spools in robot arm 16. Preferred rotaryvacuum communication spools are described in U.S. Pat. No. 5,741,113 forCONTINUOUSLY ROTATABLE MULTIPLE LINK ROBOT ARM MECHANISM, which isassigned to the assignee of this application.

Piston 52 further includes an annular groove 68 that is in pneumaticcommunication with a vent (not shown) in piston rod 56. First and secondchannels 64 and 66 are connected to, respectively, drive chamber 60 andreturn chamber 62 at locations that are opened to groove 68 at thetravel limits of piston 52. Therefore, vacuum pressure in first andsecond channels 64 and 66 is reduced at the travel limits of piston 52,thereby providing signals to the vacuum controller that active contactpoint 50 is fully extended or retracted to effect proper loading ofwafer 12.

After wafer 12 is loaded onto end effector 10, active contact point 50is actuated to move wafer 12 into its gripped position. As activecontact point 50 is extended, it urges wafer 12 toward distal rest pads24 until wafer 12 is gripped within exclusion zone 30 by active contactpoint 50 and distal rest pads 24.

Proximal rest pads 26 are arranged relative to distal rest pads 24 sothat plane 36 of wafer 12 is preferably parallel to end effector 10 whengripped. This arrangement is readily achieved when the flat embodimentof proximal and distal rest pads 24 and 26 is employed. However, whenthe inclined embodiment is employed, proximal and distal rest pads 24and 26 are arranged such that the points where wafer 12 contacts padportions 42 are substantially equidistant from a center 70 of wafer 12when active contact point 50 is extended and wafer 12 is gripped. Forexample, when wafer 12 is in the position shown in FIG. 1, the padportions of distal and proximal rest pads 24 and 26 contact wafer 12 atpoints tangent to periphery 18 such that a line through the center ofeach pad portion 42 intersects center 70 of wafer 12. Wafer 12 is,therefore, laterally centered when its peripheral edge is gripped.

The location of active contact point 50 at proximal end 20 allows endeffector 10 to be lighter, stronger, and more slender than end effectorshaving moving mechanisms that may not fit between adjacent wafers 12,12A, and 12B in cassette 14. The lack of moving mechanisms furthercauses end effector 10 to produce less contamination within cassette 14.Additionally, locating active contact point 50 at proximal end 20 of endeffector 10 ensures that active contact point 50 is remote from harshconditions such as heated environments and liquids.

The close spacing of adjacent wafers 12, 12A, and 12B requires accuratepositioning of end effector 10 to enter cassette 14 and positively gripthe wafers without touching adjacent wafers.

FIGS. 5, 6A, and 6B show respective top, side, and front views of afirst embodiment of wafer edge and elevation sensors that provideaccurate wafer 12 positioning data relative to end effector 10. (Wafer12 is shown transparent to reveal underlying structures.) The sensorsare housed in first and second sensor housings 80 and 82, which togetherform three light transmission sensors, each having a fiber opticsource/receiver pair.

Two wafer edge sensors are implemented as follows. First and secondsensor housings 80 and 82 each include a light source fiber 84 and alight receiver fiber 86 that form between them a small U-shaped opening88 into which periphery 18 of wafer 12 can fit. Fibers 84 and 86 furtherinclude mutually facing light path openings 90 that form a narrow lighttransmission pathway for detecting the presence or absence of periphery18 of wafer 12. Fibers 84 and 86 extend through ferrules 92 to a lightsource/receiver module 94 that is mounted on a convenient location ofend effector 10 near its rotary connection to robot arm 16. Lightsource/receiver module 94 conventionally detects degrees of lighttransmission between fibers 84 and 86 and, thereby, accurately sensesthe positioning of periphery 18 between light path openings 90. Ofcourse, the relative positions of fibers 84 and 86 may be reversed.

One elevation sensor is implemented as follows. First sensor housing 80further includes a light source fiber 96 (shown in phantom), and secondsensor housing 82 includes a light receiver fiber 98 (shown in phantom).Fibers 96 and 98 form between them a wide opening that sights along abottom surface chord 100 of wafer 12. Fibers 96 and 98 further includemutually facing light path openings 102 that form a narrow lighttransmission pathway 104 for detecting the presence or absence of bottomsurface chord 100 of wafer 12. Fibers 96 and 98 extend through ferrules106 to light source/receiver module 94. Light source/receiver module 94conventionally detects degrees of light transmission between fibers 96and 98 and thereby accurately senses the positioning of bottom surfacechord 100 between light path openings 102. Of course, the relativepositions of fibers 96 and 98 may be reversed.

Flat 27 may be detected by separating light path openings 102 from eachother by distance greater than the length of flat 27. Flat 27 is presentif bottom surface chord 100 is sensed between light path openings 102,but periphery 18 is not sensed between one of the pairs of light pathopenings 90.

The procedure by which end effector 10 accesses wafer 12 of a knowndiameter, such as 200 mm, is described below with reference to FIGS. 2,5, 6A, and 6B.

Active contact point 50 is placed in its retracted or wafer-releasingposition.

End effector 10 is inserted in an X direction into cassette 14 between,for example, wafers 12 and 12B, until periphery 18 is sensed between atleast one pair of light path openings 90.

A controller (not shown) associated with robot arm 16 records theextension of robot arm 16 when periphery 18 is sensed, ignoring anysensed flat.

End effector 10 is retracted in the -X direction by an amount sufficientto provide clearance between wafer 12 and the edge detectors.

Robot arm 16 is moved in a Z direction until bottom surface chord 100 ofwafer 12 is sensed.

The controller records the Z elevation of the bottom surface of wafer12.

The controller computes the X distance required to reach into cassette14 at a Z elevation below the bottom surface of wafer 12 so distal andproximal rest pads 24 and 26 clear wafers 12 and 12B. End effector 10 inthis position defines a space between wafer 12 and the proximal anddistal pad wafer contacting surfaces of the distal and proximal padportions of support surface 10 s.

The controller also accounts for:

-   -   1) a radial distance offset and an elevation distance offset of        distal rest pads 24 relative to the Z elevation of light        transmission pathway 104, and    -   2) the radial distance end effector 10 was retracted after        sensing periphery 18.

The controller moves end effector 10 in the X direction into cassette 14and elevates in the Z direction to eliminate the space between andthereby contact wafer 12 on rest pads 24 and 26.

Active contact point 50 is actuated toward a wafer-securing position tomove wafer 12 along the proximal and distal pad wafer contactingsurfaces of the distal and proximal pad portions of support surface 10s. This movement of wafer 12 urges its peripheral edge into the includedangle between pad and backstop portions 32 and 34 of distal rest pads24, thereby gripping wafer 12.

End effector 10 withdraws wafer 12 in the -X direction from cassette 14.

FIGS. 7 and 8 show a second embodiment of a spatula-shaped end effector110 of this invention for transferring semiconductor wafers, such aswafer 12 (shown transparent to reveal underlying structures), to andfrom wafer cassette 14 (not shown in this view). End effector 110 issimilar to end effector 10 but is further adapted to sense the bottomsurface of a wafer stored in wafer cassette 14 without protruding intothe cassette. FIG. 8 shows that end effector 110 is particularly adaptedfor retrieving and replacing wafer 12 from among closely spaced apartwafers, such as wafers 12, 12A, and 12B, which are shown as they mightbe stacked in wafer cassette 14.

End effector 110 is operably attached to robot arm 16. In general, endeffector 110 senses the bottom surface of wafer 12 before entering wafercassette 14 to retrieve wafer 12 from between wafers 12A and 12B. Endeffector 110 is then finely positioned by robot arm 16 and actuated togrip periphery 18 of wafer 12, remove wafer 12 from cassette 14, andtransfer wafer 12 to a processing station (not shown) for processing.End effector 110 may then, if necessary, reinsert wafer 12 into cassette14, release wafer 12, and withdraw from cassette 14.

End effector 110 is operably coupled to robot arm 16 at a proximal end120 and extends to a distal end 122. End effector 110 receives wafer 12between proximal end 120 and distal end 122 and preferably includes on asupport surface 110 s at least two and, more preferably, four arcuaterest pads upon which wafer 12 is initially loaded. Two distal arcuaterest pads 124 are located at, or adjacent to, distal end 122 of endeffector 110; and at least one, but preferably two proximal arcuate restpads 126 are located toward proximal end 120. Distal and proximalarcuate rest pads 124 and 126 may have an angular extent greater thanflat 27, which is shown, by way of example only, positioned betweenproximal rest pads 126. Of course, wafer 12 may have a differentorientation from that shown.

Arcuate rest pads 124 and 126, whether separated as shown, or joinedinto a single rest pad, have an angular extent greater than any featureon wafer 12 to guarantee that wafer 12 is sufficiently gripped, whetherflatted or not, and only within exclusion zone 30. Like rest pads 24 and26, rest pads 124 and 126 may be made of various materials, but thepreferred material is peek.

FIG. 9 shows the embodiment of distal arcuate rest pads 124 that issuitable for use with flatted or nonflatted wafers. Distal arcuate restpads 124 include an inclined pad portion 132 and a backstop portion 134.Referring also to FIG. 4, inclined pad portion 132 is inclined away fromwafer 12 at rest pad angle 44 of about 3 degrees relative to plane 36,and backstop portion 134 is inclined toward wafer 12 at backstop angle38 of up to about 3 degrees. Inclined pad portion 132 has a length 140that is a function of the depth of exclusion zone 30, but is preferablyabout 3 mm long. As before, wafer 12 typically has a substantiallyrounded peripheral edge and contacts arcuate rest pads 124 by wafer edgecontact (and perforce only within exclusion zone 30). Of course, theperipheral edge need not be rounded. Wafer 12 is gripped by urging itinto the included angle formed between inclined pad portion 132 andbackstop portion 134.

Distal arcuate rest pads 124 have a height 148 that substantiallyreaches but does not extend beyond the top surface of wafer 12.

Referring again to FIG. 7, proximal arcuate rest pads 126 are similar todistal arcuate rest pads 124 except that each rest pad 126 does notnecessarily require a backstop portion and its pad portion has a lengthof about twice that of length 140.

End effector 110 further includes an active contact point 150 that islocated at proximal end 120 of end effector 110 and between proximalarcuate rest pads 126. Alternatively, proximal end contact point 150 isformed as part of a proximal rest pad 126. Active contact point 150 ismovable between a retracted wafer-loading position (not shown) and theextended wafer-gripping position shown.

Referring again to FIG. 8, a second embodiment of an active contactpoint actuating mechanism 151 is shown employed with end effector 110.Active contact point 150 is operatively connected to a piston 152 forreciprocation between retracted and extended positions. In thisembodiment, piston 152 reciprocates within a bore 154 and is urged by abiasing device or spring 155 to extend active contact point 150 and by avacuum pressure to retract active contact point 150. Active contactpoint 150 is connected to piston 152 by a piston rod 156 that extendsthrough an annular airtight seal 158. Bore 154 includes an end cap 159that forms one wall of a vacuum chamber 160, the other wall of which ismovably formed by piston 152. Vacuum chamber 160 is in pneumaticcommunication with a vacuum pressure source (not shown) through a vacuumfeedthrough 162 and a vacuum channel 164. Spring 155 presses against theface of piston 152 to extend active contact point 150 to thewafer-gripping position, whereas the vacuum pressure acts through vacuumchamber 160 against the face of piston 152 to overcome the spring forceand retract active contact point 150 to the wafer-releasing position.

In the second embodiment, active contact point 150 is urged againstwafer 12 with a force determined solely by spring 155. Spring 155 issupported between recesses 166 in piston 152 and end cap 159. The vacuumpressure source is routed to vacuum channel 164 through rotary vacuumcommunication seals or spools in robot arm 16. Thus, in the event ofloss of vacuum pressure or other facilities, end effector 110 operatesin a failsafe manner with spring 155 applying a biasing force thatcauses active contact point actuating mechanism 151 to attain itswafer-securing position to hold wafer 12 in its gripped position.

Actuating mechanism 151 further includes a vent 168 in pneumaticcommunication with the atmosphere to allow free movement of piston 152within the portion of bore 154 not in pneumatic communication with thevacuum pressure source. Actuating mechanism 151 is made “vacuum tight”by O-ring seals 170 surrounding end cap 159 and vacuum feedthrough 162and by an annular moving seal 172 surrounding piston 152. O-ring bumperseals 174 fitted to the faces of piston 152 absorb contact shockspotentially encountered by piston 152 at the extreme ends of its travel.

After wafer 12 is loaded onto end effector 110, active contact point 150is actuated to move wafer 12 into its gripped position. As activecontact point 150 is extended by spring 155, it urges wafer 12 towarddistal arcuate rest pads 124 until wafer 12 is gripped by wafer edgecontact (and perforce within exclusion zone 30) by active contact point150 and distal arcuate rest pads 124. Active contact point 150 includesan inwardly inclined face portion 176 to impart to wafer 12 a motiveforce component that urges wafer 12 toward proximal arcuate rest pads126, thereby firmly gripping the peripheral edge of wafer 12.

Proximal arcuate rest pads 126 are arranged relative to distal arcuaterest pads 124 so that the plane of wafer 12 is preferably parallel toend effector 110 when gripped.

In a manner similar to end effector 10, the location of active contactpoint 150 at proximal end 120 allows end effector 110 to be lighter,stronger, and more slender than end effectors having moving mechanismsthat may not fit between adjacent wafers 12, 12A, and 12B in cassette14. The lack of moving mechanisms between its proximal end 120 anddistal end 122 further causes end effector 110 to produce lesscontamination within cassette 14. Moreover, unlike end effector 10,which is actuated by two vacuum lines, end effector 110 requires onlyone vacuum line for actuation. Of course, end effector 10 could befitted with actuating mechanism 151.

The close spacing of adjacent wafers 12, 12A, and 12B requires accuratepositioning of end effector 110 to enter cassette 14 and positively gripthe wafers without touching adjacent wafers.

FIGS. 7, 10, and 11 show respective top, end, and bottom views of asecond embodiment of wafer edge and elevation sensors that provideaccurate wafer 12 positioning data relative to end effector 110. Thewafer edge sensors are housed in first and second sensor housings 180and 182, each having a fiber optic source/receiver pair forming a lighttransmission sensor in each housing. The elevation sensor is housed indistal end 122 of end effector 110.

Two wafer edge sensors are implemented as follows. First and secondsensor housings 180 and 182 each include light source fiber 84 and lightreceiver fiber 86, as in end effector 10, that form between them a smallU-shaped opening 88 into which periphery 18 of wafer 12 can fit. Asbefore, fibers 84 and 86 include mutually facing light path openingsthat form a narrow light transmission pathway for detecting the presenceor absence of periphery 18 of wafer 12. The two wafer edge sensors areseparated from each other by a distance 183 greater than the length offlat 27 so that a flatted wafer can be detected when only one of the twowafer edge sensors detects periphery 18 of wafer 12. Of course, wafer 12must be appropriately oriented in cassette 14 to detect flat 27.

The elevation sensor is implemented as follows. Unlike the firstembodiment, first and second sensor housings 180 and 182 do not includelight source fiber 96 and light receiver fiber 98. Rather in thisembodiment, light source fiber 96 is routed through a first channel 184formed in the bottom surface of end effector 110 and running betweenproximal end 120 and a first distal tine 188 proximal to distal end 122of end effector 110. In like manner, light receiver fiber 98 is routedthrough a second channel 186 formed in the bottom surface of endeffector 110 and running between proximal end 120 and a second distaltine 190 proximal to distal end 122 of end effector 110. Distal tines188 and 190 are widely spaced apart across a gap 191 that forms a reliefregion for certain types of processing equipment, such as waferprealigners.

Fibers 96 and 98 terminate in mutually facing light path openings 192and 194 formed in distal tines 188 and 190. Fibers 96 and 98 formbetween them a wide opening that sights along a bottom surface chord 200of, for example, wafer 12A. Mutually facing light path openings 192 and194 form a narrow light transmission pathway 202 for detecting thepresence or absence of bottom surface chord 200 of wafer 12A. In endeffector 110, light transmission pathway 202 extends beyond the portionof distal end 122 that would first contact wafer 12, thereby furtherproviding an obstruction sensing capability. As before, lightsource/receiver module 94 conventionally detects degrees of lighttransmission between fibers 96 and 98 and, thereby, accurately sensesthe positioning of bottom surface chord 200 between light path openings192 and 194. Of course, the relative positions of fibers 96 and 98 maybe reversed.

The procedure by which end effector 110 accesses a predetermined waferfrom among closely spaced apart wafers in a cassette, is described belowwith reference to FIGS. 7, 8, and 10.

Active contact point 150 is placed in its retracted position.

End effector 110 is moved in an X direction toward cassette 14 untiltines 188 and 190 are adjacent to, but not touching, a predictedposition for any wafer 12 in cassette 14.

End effector 110 is then scanned in a Z direction such that lighttransmission pathway 202 intersects the bottom surface chord 200 of anywafer in cassette 14 and, additionally, detects any obstructionprojecting from cassette 14 toward end effector 110.

The controller (not shown) records the Z elevations of the bottomsurfaces of any wafers and obstructions detected.

Robot arm 16 is moved to a Z elevation calculated to access apredetermined wafer, such as wafer 12A, while also providing clearancefor end effector 110 between adjacent wafers.

The following optional operations may be performed:

-   -   End effector 110 may be optionally moved in an X direction        toward cassette 14 until tines 188 and 190 are adjacent to, but        not touching, wafer 12A. In this position, light transmission        pathway 202 should be adjacent to bottom surface chord 200 of        wafer 12A;    -   robot arm 16 is optionally moved in a Z direction until bottom        surface chord 200 of wafer 12A is sensed;    -   the controller optionally verifies the previously sensed Z        elevation of the bottom surface of wafer 12A; and    -   robot arm 16 is optionally moved in a -Z direction to provide        clearance for end effector 110 between adjacent wafers.

End effector 110 is inserted in an X direction into cassette 14 betweenadjacent wafers until periphery 18 is sensed by at least one wafer edgesensor.

The controller moves end effector 10 in the Z direction by an amountcalculated to contact wafer 12A on arcuate rest pads 124 and 126.

Active contact point 150 is actuated to urge wafer 12A into the includedangle between pad and backstop portions 132 and 134 of distal arcuaterest pads 124, thereby gripping wafer 12A. (In FIG. 7, the gripped waferis shown as wafer 12.)

End effector 110 withdraws wafer 12A in the -X direction from cassette14.

End effector 110 combines a very thin Z-direction profile and accuratewafer position sensing to enable clean, rapid, and secure movement ofvery closely spaced apart wafers in a cassette.

FIGS. 12, 13, and 14 show a third embodiment of a preferred fork-shapedend effector 210 of this invention for transferring semiconductorwafers, such as wafer 12 (shown transparent to reveal underlyingstructures), to and from wafer cassette 14 (not shown in these views).End effector 210 is similar to end effectors 10 and 110 but furtherincludes a position sensing active contact point actuating mechanism212, and deletes the proximal end edge and elevation sensors. Rather,end effector 210 employs distal end sensors 214 to accomplish variouswafer sensing measurements. Distal end sensors 214 are implementedsimilarly to the elevation sensor generating light transmission pathway202 as shown in FIGS. 7 and 10.

FIG. 13 shows that end effector 210 is particularly suited forretrieving and replacing wafer 12 from among closely spaced apartwafers, such as wafers 12, 12A, and 12B, which are shown as they mightbe stacked in wafer cassette 14.

FIG. 14 shows end effector 210 operably coupled to robot arm 16 at aproximal end 216 and extending to forked distal ends 218 and 220. Endeffector 210 receives wafer 12 between proximal end 216 and forkeddistal ends 218 and 220 and preferably includes at least two and, morepreferably, four arcuate rest pads upon which wafer 12 is initiallyloaded. A distal arcuate rest pad 124 is located at, or adjacent to,each of forked distal ends 218 and 220; and at least one, but preferablytwo proximal arcuate rest pads 126 are located toward proximal end 216.End effector 210 also includes an active contact point 222 that islocated at proximal end 216 of end effector 210 and between proximalarcuate rest pads 126.

Referring to FIGS. 12 and 13, position sensing active contact pointactuating mechanism 212 is a third embodiment of the active contactpoint actuating mechanism. As in the second embodiment, active contactpoint 222 is operatively connected to piston 152 for reciprocationbetween fully retracted, fully extended, and intermediate positions.Piston 152 moves within bore 154 and is urged by a spring (FIG. 8) toextend active contact point 222 and by a vacuum pressure to retractactive contact point 222. Active contact point 222 is connected topiston 152 by piston rod 156 that extends through annular airtight seal158. Bore 154 includes end cap 159 that forms one wall of vacuum chamber160, the other wall of which is movably formed by piston 152. Vacuumchamber 160 is in pneumatic communication with the vacuum pressuresource (not shown) through vacuum feedthrough 162 and vacuum channel164. The spring presses against the face of piston 152 to extend activecontact point 222 to wafer-gripping and fully extended positions,whereas the vacuum pressure acts through vacuum chamber 160 against theface of piston 152 to overcome the spring force and retract activecontact point 222 to wafer-releasing and fully retracted positions.

Actuating mechanism 212 further includes vent 168 in pneumaticcommunication with the atmosphere to allow free movement of piston 152within the portion of bore 154 not in pneumatic communication with thevacuum pressure source. Actuating mechanism 212 is made “vacuum tight”by O-ring seals 170 surrounding end cap 159 and vacuum feedthrough 162,and by an annular moving seal 172 surrounding piston 152.

Unlike the first and second embodiments, actuating mechanism 212 furtherincludes a position indicating shaft 224 attached to piston 152 andextending axially through an annular seal 226 in end cap 159. A pair ofoptical interrupter switches 228 and 230 are mounted to a circuit board232 positioned just behind end cap 159 such that, depending on theposition of indicating shaft 224, it interrupts a pair of light beams234 and 236 in respective optical interrupter switches 228 and 230.

Optical interrupter switches 228 and 230 sense positions of activecontact point 222 corresponding to a retracted position region, a safegripping operation region, and an extended position region. (FIGS. 12and 13 show active contact point 222 in a fully extended position.)

The retracted position region ensures that wafer 12 is not gripped andis sensed when position indicating shaft 224 interrupts both of lightbeams 234 and 236.

The safe gripping operation region is a range of active contact point222 positions within which wafer loading, gripping, or unloadingoperation can be safely carried out and is sensed when positionindicating shaft 224 interrupts light beam 236 but not light beam 234.Moreover, when active contact point 222 is extended and comes to rest inthe safe gripping operation region, proper wafer gripping is verified.

The extended position region is a range of active contact point 222positions within which wafer 12 is not gripped and is sensed whenposition indicating shaft 224 interrupts neither of light beams 234 and236.

Optical interrupter switches 228 and 230 are in electrical communicationwith the above-referenced controller. The controller coacts with thevacuum pressure source actuating piston 152 to pulse or pressureregulate the amount of vacuum pressure and, thereby, control thepositions of active contact point 222. Of course, various other forms ofcontrollable motive forces may be employed to position active contactpoint 222.

In an operational example, active contact point 222 is moved to the safegripping operation region and a wafer 12 is loaded into end effector210. After wafer 12 is loaded, active contact point 222 is actuated tomove wafer 12 into its gripped position. As active contact point 150 isextended, it urges wafer 12 up inclined pad portions 132 of distalarcuate rest pads 124 away from support surface 110 s and down theinclined proximal pad portions of proximal arcuate rest pads 126 towardsupport surface 110 s until wafer 12 is gripped. Active contact point222 must be sensed in the safe gripping operating region to ensure thatwafer 12 is properly gripped.

Wafer 12 is released by retracting active contact point 222 to theretracted position region as sensed by position indicating shaft 224interrupting both of light beams 234 and 236. When wafer 12 is released,it slips back on inclined pad portions 132 of distal arcuate rest pads124, thereby providing sufficient clearance between wafer 12 andbackstop portion 134 for a safe Z-axis elevation move and retrieval ofend effector 210.

FIG. 14 shows a top view of the third embodiment of end effector 210 inwhich the wafer edge sensors of end effectors 10 and 110 have beenremoved. Distal end sensors 214 of end effector 210 are housed in forkeddistal ends 218 and 220. Distal end sensors 214 are implemented asfollows. A light source fiber is routed through a first channel 238(shown in phantom lines) formed in the bottom surface of end effector210 and running between proximal end 216 and forked distal end 218. Inlike manner, a light receiver fiber is routed through a second channel240 (shown in phantom lines) formed in the bottom surface of endeffector 210 and running between proximal end 216 and forked distal end220. Forked distal ends 218 and 220 are widely spaced apart across a gap242 that forms a relief region for certain types of processingequipment, such as wafer prealigners.

The light fibers terminate in mutually facing light path openings (notshown) formed in forked distal ends 218 and 220. The fibers form betweenthem a wide opening that sights along the peripheral edge or the bottomsurface chord of a wafer. The mutually facing light path openings form anarrow light transmission pathway 244 for detecting the presence orabsence of the periphery or bottom surface chord of a wafer. Lighttransmission pathway 244 extends beyond the portion of forked distalends 218 and 220 that would first contact a wafer, thereby furtherproviding an obstruction sensing capability. As before, lightsource/receiver module 94 conventionally detects degrees of lighttransmission between the fibers and, thereby, senses any objects thatinterrupt light transmission pathway 244.

End effector 210 employs distal end sensors 214 to accomplish variouswafer sensing measurements including sensing wafer protrusion from acassette, wafer edge sensing, wafer top and bottom chord sensing, wafertilt, wafer center determination, wafer thickness, center-to-centerdistance between the wafer and the robot arm rotational axis, anddetermining the end effector centroid. The sensing measurements aredescribed with reference to light transmission pathway 244 of endeffector 210, but they can also be accomplished with light transmissionpathway 202 of end effector 110.

Three alternative wafer positions are shown in FIG. 14. Wafer 12 (shownin phantom) is shown gripped by end effector 210, wafer 12A (shown insolid lines) is shown in a wafer edge sensing position, and wafer 12B(shown in phantom) is shown in a wafer chord sensing position.

Sensing wafer 12B protrusion from a cassette (not shown) entailsstepping robot arm 16 up and down in the Z-axis direction while alsomoving end effector 210 in the X-axis direction until wafer 12B isdetected. Prior robot arm systems typically employed a dedicatedprotrusion sensor. Robot arm 16 X- and Z-axis movements are preferablyin a fine resolution mode.

After light transmission pathway 244 is interrupted, indicating detectedpresence of wafer 12B, end effector 210 can find wafer 12B top andbottom surfaces by moving end effector 210 downward in the Z-axisdirection until a top surface chord of wafer 12B interrupts lighttransmission pathway 244. End effector 210 continues moving downwarduntil light transmission pathway 244 is restored. This point representssensing a bottom surface chord of wafer 12B. End effector 210 is thenmoved to a Z-axis position midway between the points of interruption andrestoration of light transmission pathway. This Z-axis positionrepresents the approximate midpoint of wafer 12B thickness. Whilemaintaining this Z-axis position, end effector 210 is retracted in theX-axis direction until light transmission pathway 244 is restored,indicating that periphery 18 of the wafer has been detected. Wafer 12Ais shown in this position.

When end effector 210 is at the edge detection point represented bywafer 12A and because the radius of wafer 12A is known, the controllerand position encoders associated with robot arm 16 can determine theX-axis direction distance to a center 246 of wafer 12A and a downwardZ-axis distance required to provide clearance between the bottom surfaceof wafer 12A and end effector 210. Knowing the clearance is necessarywhen placing and retrieving wafers from the cassette because the wafersare not necessarily parallel to end effector 210 and distances betweenadjacent wafers in the cassette can be tight.

End effector 210 further includes a controllable supination angle 248,which is the tilt angle about the X-axis of end effector 210 relative toa Y-axis. Wafers stacked in a cassette would have their major surfaceplanes at a predetermined tilt angle, preferably zero degrees, thatshould be matched by supination angle 248 of end effector 210. Todetermine whether supination angle 248 is level with the tilt angle of awafer, robot arm 16 moves end effector 210 up and down in the Z-axisdirection while dithering its supination angle 248 until a minimum waferthickness is computed, which indicates that end effector 210 and thewafer are in the same datum plane. Robot arm systems can be equippedwith two end effectors or multiple arms (see FIGS. 15A and 15B for dualarm robot). The technique described above for a controllable supinationangle can be extended to such multiple end effector systems by using asingle wafer as a reference to determine the X, Y, and Z dimensionoffsets among them.

Light transmission pathway 244 may also be employed to determine theX-axis position of a wafer in the cassette or on a prealigner. Thisdetermination entails finding the minimum distance between a shoulderaxis 316 of robot arm 16 and the front of a wafer, for example, wafer12B. Finding this minimum distance then provides the corresponding robotarm extension and angle values. The determination entails angularlydisplacing robot arm 16 such that light transmission pathway 244intersects wafer 12B at two different chord positions, such as chordpositions 254 and 256. There is a variety of available search routinesthat can be used to compute this minimum distance. This distancedetermination is accomplished without any of the teaching fixturesrequired by prior robot arms and end effectors. If multiple endeffectors 210 are employed, the foregoing procedure can be repeatedtogether with determining any Z-axis elevation difference between them.

Referring to FIG. 5, it should be noted that the U-shaped edge detectingsensors in housing 80 and 82 are useful for determining certainparameters of a flatless 300 mm wafer. For instance, the edge detectingsensors can be employed to determine the center-to-center distancebetween shoulder axis 316 of robot arm 16 and a wafer center while thewafer is in the cassette or end effector 10 is positioned beneath thewafer. Of course, the Z-axis dimension of U-shaped openings 88 (FIG. 6A)presents a potential spacing problem.

Referring again to FIG. 14, light transmission pathway 244 may also beused in combination with the supination capability of end effector 210to determine whether a centroid 262 of end effector 210 is axiallyaligned with center 252 of wafer 12B. Ideally, centroid 262 is coaxialwith the center of gripped wafer 12 and lies on an imaginary lineextending between shoulder axis 316 and center 252 of wafer 12B.However, manufacturing tolerances and the positionings of featurescreating light transmission pathway 244 may cause a calculated positionof centroid 262 to be offset from the supination axis of rotation.Determining whether centroid 262 is offset or coincident entailscarrying out the above-referenced robot arm 16 movements and distancecalculations to determine the location of center 252 of wafer 12B,rotating end effector 210 through a supination angle 248 of 180 degreesand repeating the center 252 location calculation. If the centroid isoffset, the calculated location of center 252 will be in a mirror imageposition on the opposite side of the supination axis of rotation. Thecorrect location for centroid 262 is determined by averaging the twocalculated locations for center 252 of wafer 12B.

The above-described embodiments are merely illustrative of theprinciples of the invention. Various modifications and changes may bemade thereto by those skilled in the art that will embody the principlesof the invention and fall within the spirit and scope thereof. Forexample, skilled workers will understand that the pistons may beactuated by alternative power sources, such as, for example, by apulsing solenoid that slows the pistons as wafer 12 is secured. Electricsignals may be employed to drive and monitor the positioning of thepistons. The pistons may also be pneumatically operated and monitored,such as in applications where the end effectors are submerged in aliquid. The end effectors may be forked or otherwise include a cutout orbe shaped to avoid obstacles, such as a prealigner hub. The sensorspreferably employ laser beams from light-emitting diodes and diodelasers, but may also employ incandescent, infrared, and other radiationsources. Moreover, the end effector is usable for handling various typesof specimens other than semiconductor wafers, such as compact diskettesand computer memory discs.

FIGS. 15A and 15B and FIGS. 16 and 17 show a type of multiple link robotarm system 308 to which end effector 210 is mountable. FIGS. 18A and 18Bpresent in conjunction with pertinent mathematical expressionscharacterizing robot arm displacement an example of positioning robotarm mechanism 308 to demonstrate the manipulation of the linear andangular displacement values necessary to compute the parametersassociated with the various wafer sensing measurements described above.U.S. Pat. No. 5,765,444 provides a detailed description of theconstruction and operation of this type of robot arm system.

FIGS. 15A and 15B are respective side elevation and plan views of atwo-arm, multiple link robot arm system 308 mounted on and through anaperture in the top surface of a support table 309. With reference toFIGS. 15A and 15B, two similar but independently controllable three-linkrobot arm mechanisms 310L and 310R are rotatably mounted at oppositeends of a torso link 311, which is mounted to the top surface of a basehousing 312 for rotation about a central or torso axis 313. Because theyare mirror images of each other, robot arm mechanisms 310L and 310R havecorresponding components identified by identical reference numeralsfollowed by the respective suffices “L” and “R”. Accordingly, thefollowing discussion is directed to the construction and operation ofonly robot arm mechanism 310R but is similarly applicable to robot armmechanism 310L.

Robot arm mechanism 310R comprises an upper arm 314R mounted to the topsurface of a cylindrical spacer 315R, which is positioned on theright-hand end of torso link 311 for rotation about a shoulder axis316R. Cylindrical spacer 315R provides room for the motors and certainother components of robot arm mechanism 310R, as will be describedbelow. Upper arm 314R has a distal end 318R to which a proximal end 320Rof a forearm 322R is mounted for rotation about an elbow axis 324R, andforearm 322R has a distal end 326R to which a proximal end 328R of endeffector or hand 210R is mounted for rotation about a wrist axis 332R.Hand 210R is equipped at its distal end 334R with a fluid pressureoutlet 336R that preferably applies vacuum pressure supplied to robotarm mechanism 310R at an inlet 338 to vacuum channel 164 to securelyhold semiconductor wafer 12, a compact disk, or other suitable specimen(not shown) in place on hand 210R. As will be described in detail later,each of upper arm 314R, forearm 322R, and hand 210R is capable ofcontinuous rotation about its respective shoulder axis 316R, elbow axis324R, and wrist axis 332R.

FIG. 16 shows the link components and associated mechanical linkage ofrobot arm mechanism 310R. With reference to FIG. 16, robot arm mechanism310R is positioned by first and second concentric motors 350R and 352Rthat operate in response to commands provided by a motor controller 354(FIGS. 18A and 18B). First motor 350R rotates forearm 322R about elbowaxis 324R, and second motor 352R rotates upper arm 314R about shoulderaxis 316R.

More specifically, first motor 350R rotates a forearm spindle 356R thatextends through an aperture in upper arm 314R and terminates in an upperarm pulley 358R. A post 360R extends upwardly at distal end 318R ofupper arm 314R through the center of a bearing 362R that is mounted to abottom surface 364R of forearm 322R at its proximal end 320R. Post 360Ralso extends through an aperture in forearm 322R and terminates in aforearm pulley 366R. An endless belt 368R connects upper arm pulley 358Rand the outer surface of bearing 362R to rotate forearm 322R about elbowaxis 324R in response to rotation of first motor 350R.

Second motor 352R rotates an upper arm spindle 380R that is mounted to abottom surface 382R of upper arm 314R to rotate upper arm 314R aboutshoulder axis 316R. Coordinated operation of first and second motors350R and 352R in conjunction with the mechanical linkage described belowcauses hand 210R to rotate about shoulder axis 316R. A post 384R extendsupwardly through the center of a bearing 386R that is mounted to abottom surface 388R of hand 210R. An endless belt 390R connects forearmpulley 366R to the outer surface of bearing 386R to rotate hand 210Rabout shoulder axis 316R in response to the coordinated rotationalmotions of motors 350R and 352R.

The mechanical linkage coupling upper arm 314R and forearm 322R forms anactive drive link and a passive drive link. The active drive linkincludes belt 368R connecting upper arm pulley 358R and the outersurface of bearing 362R and causes forearm 322R to rotate in response torotation of first motor 350R. The passive drive link includes belt 390Rconnecting forearm pulley 366R and the outer surface of bearing 386R andcauses hand 210R to rotate about wrist axis 332R in response to rotationof forearm 322R about elbow axis 324R. Rotation of hand 210R can also becaused by a complex interaction among the active and passive drive linksand the rotation of upper arm 314R in response to rotation of secondmotor 352R.

A third or torso motor 392 rotates a torso link spindle 394 that ismounted to a bottom surface of torso link 311, to which robot armmechanism 310R is rotatably mounted. A main ring 396 supports a bearingassembly 398 around which spindle 394 rotates. Motor 392 is capable of360 degree continuous rotation about central axis 313 and therefore can,in cooperation with robot arm mechanism 310R, move hand 210R along anirregular path to any location within the reach of hand 210R.

Motor controller 54 (FIGS. 18A and 18B) controls motors 350R and 352R intwo preferred operational states to enable robot arm mechanism 310R toperform two principal motion sequences. The first motion sequencechanges the extension or radial position of hand 210R, and the secondmotion sequence changes the angular position of hand 210R relative toshoulder axis 316R. FIG. 17 is a useful diagram for showing the twomotion sequences.

With reference to FIGS. 16 and 17, in the first operational state, motorcontroller 354 causes first motor 350R to maintain the position offorearm spindle 356R and second motor 352R to rotate upper arm spindle380R. The non-rotation of first motor 350R maintains the position ofupper arm pulley 38R, and the rotation of upper arm spindle 380R bysecond motor 352R rotates upper arm 314R about shoulder axis 316R,thereby causing rotation of forearm 322R about elbow axis 324R andcounter-rotation of hand 210R about wrist axis 332R. Because the ratioof the diameters of upper arm pulley 358R and the outer surface ofbearing 362R are 4:2 and the ratio of the diameters of forearm pulley366R and the outer surface of bearing 386R is 1:2, the rotation of upperarm 314R in a direction specified by P₂ shown in FIG. 17 will cause hand210R to move along a straight line path 400. (The diameters of forearmpulley 366R and the outer surface of bearing 386R are one-half of thediameters of, respectively, the outer surface of bearing 362R and upperarm pulley 358R to streamline the sizes and shapes of forearm 322R andhand 210R.)

Whenever upper arm 314R rotates in the clockwise direction specified byP₂, hand 210R extends (i.e., increases radial distance from shoulderaxis 16R) along path 400. Whenever upper arm 314R rotates in thecounter-clockwise direction specified by P₂, hand 210R retracts (i.e.,decreases radial distance from shoulder axis 316R) along path 400.Skilled persons will appreciate that robot arm mechanism 310 in a mirrorimage configuration of that shown in FIG. 17 would extend and retract inresponse to upper arm 314 rotation in directions opposite to thosedescribed. FIG. 15B shows that when robot arm mechanism 310R isextended, axes 313, 316R, 324R, and 332R are collinear.

In the second operational state, motor controller 352R causes firstmotor 350R to rotate forearm spindle 356R in the direction specified byP₁ and second motor 352R to rotate upper arm spindle 380R in thedirection specified by P₂. In the special case in which motors 350R and352R are synchronized to rotate in the same direction by the same amountof displacement, hand 210R is only angularly displaced about shoulderaxis 316R. This is so because the rotation of forearm 322R about elbowaxis 324R caused by the rotation of first motor 350R and the rotation ofhand 330R about wrist axis 332R caused by rotation of second motor 352Rand the operation of the passive drive link offset each other to produceno net rotation about elbow axis 324R and wrist axis 332R. Thus, hand210R is fixed radially at a point along path 400 and describes acircular path as only upper arm 314R rotates about shoulder axis 316R.By application of kinematic constraints to achieve a desired travel pathfor hand 210, motor controller 354 can operate first and second motors350R and 352R to move robot arm mechanism 310R along non-radial straightline paths, as will be further described below.

Skilled persons will appreciate that to operate robot arm mechanism310R, first and second motors 350R and 352R are coupled by eitherrotating both of them or grounding one while rotating the other one. Forexample, robot arm mechanism 310R can be operated such that forearm 322Rrotates about elbow axis 324R. Such motion would cause hand 210R todescribe a simple spiral path between shoulder axis 316R and the fullextension of hand 210R. This motion is accomplished by fixing theposition of shoulder 314R and operating motor 350R to move forearm 322R.

Motor controller 354 controls the operation of torso motor 392 andtherefore the rotation of torso link 311 in a direction specified by P₃independently of the operational states of motors 350R and 352R.

The angular positions of motors 350R and 352R are tracked by separateglass scale encoders (not shown). Each of the encoders typicallyincludes an annular diffraction grating scale and a lightsource/detector subassembly (not shown). Such glass scale encoders areknown to skilled persons. The angular position of motor 392 is trackedby a glass scale the encoder of a type similar to the encoders formotors 350R and 352R.

FIG. 18A is a diagram that specifies a local coordinate axis frame whoseaxes are defined by the orientation of a semiconductor wafer cassette168 _(r) and its location relative to shoulder axis 316R. With referenceto FIG. 18A, the following description sets forth the mathematicalexpressions from which are derived the command signals controller 354uses to retrieve from cassette 168 _(r) a wafer 170 _(r) along a vectorperpendicular to the opening of cassette 168 _(r). (Skilled persons willappreciate that similar mathematical expressions can be used fordifferent drive ratios from the above-stated drive ratio on which thisexample is based.)

The following parameters are pertinent to the derivation of the path oftravel of hand 210:

-   -   Θ_(S)=angle of motor 352R    -   Θ_(E)=angle of motor 350R    -   r=distance between shoulder axis 316R and elbow axis 324R and        distance between elbow axis 324R and wrist axis 332R    -   β=angle between upper arm 314R and forearm 322R    -   p=length of hand 210R    -   E=2r=extension of robot arm    -   R_(i)=reach of robot arm (i.e., its radius measured from        shoulder axis 316R to the center 172 _(r) of wafer 170 _(r)        positioned on hand 210R).

Application of the law of cosines provides the following expressions forR_(i): $\begin{matrix}\begin{matrix}{R_{i} = {p + \sqrt{\left( {r^{2} + r^{2} - {2r^{2}\cos\quad\beta}} \right)}}} \\{= {p + {\sqrt{2}r{\sqrt{\left( {1 - {\cos\quad\beta}} \right)}.}}}}\end{matrix} & (1)\end{matrix}$

For β=0, equation (1) provides that R_(i)=p and x=0, y=0, Θ_(S)=Θ_(SR),ΘE=Θ_(ER). The quantities Θ_(SR) and Θ_(E R) represent reference motorangles. The motor angles may be expressed as Θ_(S)=Θ_(SR)+ΔΘ_(SR),Θ_(E)=Θ_(ER)+ΔΘ_(ER). The angle β may be expressed asβ=2(ΔΘ_(SR)−ΔΘ_(ER)) because of the construction of the mechanicallinkages of robot arm mechanism 310R. This equation relates the angle βto changes in the motor angles.

To retrieve wafer 170 _(r) from cassette 168 _(r) along a straight linepath, the displacement along the X-axis equals X₀, which is a constant.Thus, X(t)=X₀. The quantity X(t) can be expressed as a function of thelengths of the X-axis components of its links:X(t)=r cos Θ₁ +r cos Θ₂ +p cos Θ_(p),  (2)in which

-   -   Θ₁=angle of upper arm 314R    -   Θ₂=angle of forearm 322R    -   Θp=angle of hand 210R.

Because upper arm 314R and forearm 322R are of the same length (r), Θ₁tracks the angle Θ_(S) of motor 352R, and hand 210R moves in a straightline, the following expressions hold: Θ₁ = Θ_(s) Θ₂ = Θ₁ + π − β$\Theta_{p} = {\Theta_{1} + {\left( \frac{\pi - \beta}{2} \right).}}$

Thus, to compute X₀, one substitutes the foregoing identities for Θ₁,Θ₂, and Θ_(p) into equation (2) for X(t) and finds: $\begin{matrix}{{X_{0} = {{r\left( {{\cos\quad\Theta_{1}} + {\cos\quad\Theta_{2}}} \right)} + {p\quad\cos\quad\Theta_{p}}}}{X_{0} = {{r\left( {{\cos\quad\Theta_{1}} + {\cos\left( \quad{\Theta_{1} + \pi - \beta} \right)}} \right)} + {p\quad\cos\quad\left( {\Theta_{1} + \frac{\pi}{2} - \frac{\beta}{2}} \right)}}}{X_{0} = {{r\left( {{\cos\quad\Theta_{1}} - {\cos\left( \quad{\Theta_{1} - \beta} \right)}} \right)} - {p\quad\sin\quad{\left( {\Theta_{1} - \frac{\beta}{2}} \right).}}}}} & (3)\end{matrix}$Equation (3) expresses the constraint that sets out the relationshipbetween the angles Θ_(S) and Θ_(E) of motors 352R and 350R operating tomove equal angular distances to achieve straight line movement of hand210R.

Skilled persons can implement constraint equation (3) by means of aservomechanism controller in any one of a number of ways. For example,to achieve high speed operation to implement a given wafer move profile,one can compute from equation (3) command signal values and store themin a look-up table for real-time use. The precomputation process wouldentail the indexing of Θ_(S) in accordance with the wafer move profileand determining from equation (3) the corresponding Θ_(E) values,thereby configuring the displacement of Θ_(S) and Θ_(E) in amaster-slave relationship.

To achieve angular displacement of hand 210R about shoulder axis 316R,controller 354 causes motors 350R and 352R to rotate in the samedirection through the desired angular displacement of hand 330R to reachthe desired destination. The linear extension of hand 330R does notchange during this move. Skilled persons will appreciate thatcomplicated concurrent linear and angular displacement move profiles ofhand 330R could be accomplished by programming controller 354 to operatemotors 350R and 352R through different angular displacements. FIG. 6Ashows a second wafer cassette 168 _(l) positioned so that the center 172_(l) of a stored wafer 170 _(l) is coincident to Y₀. The parallelarrangement of the openings of cassettes 168 _(l) and 168 _(r)demonstrates that the above expressions can be used to retrieve wafersstored in cassettes not positioned a radial distance from shoulder axis316. Robot arm mechanism 310 is not restricted to radial placement butcan accommodate any combination of distances within its reach.

FIG. 18B is a simplified block diagram showing the primary components ofcontroller 354. With reference to FIG. 18B, controller 354 includes aprogram memory 474 that stores move sequence instructions for robot armmechanism 310R. A microprocessor 476 receives from program memory 474the move sequence instructions and interprets them to determine whetherthe first or second operational state is required or whether motion ofmotor 392 is required to position torso link 311. A system clock 478controls the operation of microprocessor 476. A look-up table (LUT) 480stores corresponding values for Θ_(S) (motor 352R) and Θ_(E) (motor350R) to accomplish the straight line motion of the first operationalstate and the angular displacements of Θ_(S) and Θ_(E) to accomplish theangular motion of the second operational state. Because the rotation oftorso link 311 is independent of the motions of the robot arm mechanismsmounted to it, the overall coordination of the angular displacement ofmotor 392 with the angular displacements of motors 350R and 352R iscarried out in the move sequence instructions, not in LUT 180. Thisresults in higher speed and more accurate straight line motion becausemultiple axis servomechanism following errors and drive accuracy errorsdo not affect the straight line path of hand 210R.

Microprocessor 476 provides Θ_(S) and Θ_(E) position signals to aservomechanism amplifier 482, which delivers Θ_(S) and Θ_(E) commandsignals to motors 352R and 350R, respectively. Microprocessor 476 alsoprovides position signals to servomechanism amplifier 476 to deliver acommand signal to torso motor 392. Servomechanism amplifier 482 receivesfrom the three glass scale encoders signals indicative of the angularpositions of the respective motors 350R, 352R, and 392.

Microprocessor 476 also provides control signals to a vacuum valvecontroller 484, which causes a vacuum valve (not shown) to provide froma vacuum source (not shown) an appropriate amount of vacuum pressure tooutlet 336 in response to the need to hold a wafer on or release a waferfrom hand 210R.

It will be further obvious to those having skill in the art that manychanges may be made to the details of the above-described embodiments ofthis invention without departing from the underlying principles thereof.The scope of the present invention should, therefore, be determined onlyby the following claims.

1. A method of determining an axial alignment of a centroid of an endeffector and an effective center of a specimen held by the end effector,the end effector implemented to have a controllable supination angle ofrotation about a supination axis and operatively coupled to a robot armthat is positionable in at least two dimensions, comprising: causing theend effector to hold a specimen having an effective center; determininga first location of the effective center of the specimen; rotating theend effector holding the specimen through a supination angle of 180° anddetermining a second location of the effective center of the specimen;and determining whether the first and second locations are in mirrorimage positions on opposite sides of the supination axis of rotation toassess whether the centroid is offset from the effective center.
 2. Themethod of claim 1, further comprising calculating an average of thefirst and second locations to determine an actual location of thecentroid.
 3. The method of claim 1, in which the specimen is asemiconductor wafer.
 4. The method of claim 1, in which the end effectoris of an edge grip type.